The present invention is directed to a corrosion and grain growth resistant aluminum alloy, and particularly to an alloy composition and article having controlled levels of iron, manganese, zinc, zirconium, vanadium, titanium and other elements that is particularly adapted for use in heat exchanger applications.
In the prior art, aluminum alloys are the alloys of choice for heat exchanger applications. These alloys are selected for their desirable combination of strength, low weight, and good thermal and electrical conductivities. Some aluminum alloys may also exhibit another characteristic such as improved brazeability. Other alloys may have better formability, while others yet may exhibit good extrudability or have desirable corrosion resistance.
When an aluminum alloy component is in a corrosive environment, a common practice is to coat the aluminum alloy component with zinc for corrosion protection. This zinc is applied using a plasma spraying technique. When the alloy is in the form of a flat tube as shown in FIG. 1, it is preferred to employ the plasma spraying technique; round tubing does not lend itself to good zinc coverage when sprayed due to its curved surface.
No matter how the zinc is applied, zinc in the workplace presents a number of environmental and safety problems. When applying zinc using plasma techniques, the guns employed for such purposes are extremely loud and cause considerable noise pollution. Further, since cooling is a part of the zinc coating process, contamination of the cooling water with zinc is inevitable, and a considerable water pollution problem then exists. Zinc is also a health hazard, and OSHA has strict regulations concerning the handling of zinc and levels of zinc dust/fumes present in the workplace.
Besides the problems of zinc itself, zinc-coated aluminum products can also be problematic if the zinc coating is not uniform or is compromised. For example, a non-uniform coating or surface scratches creates local sites where the aluminum metal is exposed. These exposed local sites or cells may then corrode in a more accelerated manner than if the aluminum was not coated at all, and premature failure may occur during service.
The zinc coating processes are also expensive and contribute to the overall cost of the product being manufactured in terms of both material and operating costs. In addition, if the zinc processing equipment fails, the entire production line may be halted. As a result, productivity of the entire manufacturing line is compromised. As such, a need exists to eliminate zinc coating and provide adequate corrosion resistance in the base material itself.
Typical applications for the aluminum alloys discussed above include automotive heater cores, radiators, evaporators, condensers, charge air coolers, and transmission/engine oil coolers. One particular application that requires demanding properties is flat tubing that is employed in heat exchangers. In these applications, fin stock is arranged between stacked tubing that carries the cooling media. The tubing is situated between headers which redirect the cooling media flow between layers of tubing and which also can contain inlets and outlets. Typically, the fin stock is clad with a brazing material and the entire assembly is brazed together in a controlled atmosphere braze (CAB) process using a brazing flux. The flat tubing may be extruded as a multi-channel tubing, commonly referred to as micro-multivoid tubing (MMV tubing).
FIG. 1 shows a partial cross sectional view of a typical MMV tubing designated by the reference numeral 10. The portion of the MMV tubing depicted has sidewalls 1, each joined together by endwalls 3 (one shown). The tubing is separated into a series of voids 5 by walls 7 extending between the sidewalls 1. In certain applications, the MMV tubing is extruded at extremely light gauges, often times with sidewall thicknesses below 0.020 inches (0.51 mm).
Typically, these types of tubing are made from AA1000 series alloys such as AA1100, as well as AA3000 series alloys like 162 and 171.
One significant problem with this type of tubing is an increased number of failures in service due to corrosion. When this tubing is used in heat exchanger manufacture, stacked tubing is arranged with two sided braze clad fin stock positioned therebetween and the assembly is brazed together. During the brazing cycle, the tubing may see temperatures near its melting point and severe grain growth effects are prevalent.
Referring to FIG. 2, a cross sectional portion 9 of the MMV tubing of FIG. 1 is shown with an exemplary post-braze grain structure 20. As described above, when the MMV tubing is subjected to brazing temperature, significant grain growth can occur such that a single grain 11 can extend across the sidewall 1. With the formation of a single grain across the sidewall 1, a single grain boundary 13 can extend from the outer surface 15 of the sidewall to the inner surface 17 in one of the voids 7. The grain boundary 13 becomes an easy path for corrosion to occur through the tubing sidewall 1. This is more pronounced because the grain body is relatively much more corrosion resistant. Hence, the grain boundary is the weaker link for corrosion.
As noted above, a myriad of problems also exist in this field of aluminum alloys and heat exchanger applications when zinc is used as a coating agent.
Accordingly, a need has developed to provide an improved material for heat exchanger use, particularly MMV tubing, which overcomes the drawbacks to the prior art problems noted above. The present invention solves this need by providing an aluminum alloy composition having controlled amounts of iron, manganese, zinc, zirconium, vanadium, titanium, and other elements that effectively inhibit excessive grain growth in thin wall structures when exposed to brazing temperatures, while still maintaining extrudability and corrosion resistance.
Accordingly, it is a first object of the present invention to provide an improved aluminum alloy composition that is ideally suited for use in heat exchanger applications.
Another object of the present invention is a composition having controlled amounts of iron, manganese, zinc, zirconium, vanadium, titanium, and other elements to inhibit grain growth when the composition is exposed to brazing temperatures while still maintaining extrudability and corrosion resistance.
A still further object of the present invention is a method of brazing which utilizes an aluminum alloy composition as extruded and bare or uncoated tubing for heat exchanger applications.
Yet another object of the invention is an aluminum alloy base material that has adequate corrosion resistance but without the presence of a zinc coating.
One other object of the present invention is a method of making extruded MMV tubing using the inventive aluminum alloy composition.
Other objects and advantages of the present invention will become apparent as a description thereof proceeds.
In satisfaction of the foregoing objects and advantages, the present invention provides a novel aluminum alloy composition consisting essentially of, in weight percent;
between about 0.05 and 0.5% silicon;
up to 0.7% copper;
less than 0.01% nickel;
less than or equal to 1.0% magnesium;
up to 0.5% chromium, and
when iron is between zero and 0.09%, the composition consists essentially of:
an amount of manganese between 0.05% and 0.50%, an amount of zinc between 0.10 and 0.50%, and an amount of zirconium between 0.05 and 0.40%, and
additional amounts of one of (a), (b), or (c), wherein:
(a) is an additional amount of manganese up to 0.70%, or an amount of vanadium between 0.05 and 0.50%;
(b) is either an additional amount of manganese up to 0.70% and an amount of titanium between about 0.10 and 0.40%, or an additional amount of manganese up to 0.70% and an amount of vanadium between 0.05 and 0.50%; and
(c) is an additional amount of manganese up to 0.70%, and an amount of titanium between 0.10 and 0.40%, and an amount of vanadium between 0.05% and 0.50%;
with the balance aluminum and inevitable impurities; and when iron is between 0.09% and 0.80%, the composition includes:
an amount of manganese between 0.05% and 2.00%, and an amount of zinc between 0.10 and 0.50%; and
additional amounts of one of (a), (b), or (c), wherein:
(a) is an amount of titanium between 0.10 and 0.40%, or an amount of vanadium between 0.05 and 0.50%, or an amount of zirconium between 0.05% and 0.40%;
(b) is either an amount of titanium between about 0.10 and 0.40% and an amount of vanadium between 0.05 and 0.50%, or an amount of titanium between 0.10 and 0.40% and an amount of zirconium between 0.05 and 0.40%; or and an amount of vanadium between 0.05 and 0.50% and an amount of zirconium between 0.05 and 0.40%; and
(c) is an amount of zirconium between 0.05% and 0.40%, and an amount of titanium between 0.10 and 0.40%, and an amount of vanadium between 0.05% and 0.50%; with the balance aluminum and inevitable impurities.
In a more preferred embodiment, the alloying element ranges for the low iron embodiment are: manganese between about 0.2 and 1.0%, more preferably between about 0.3 and 0.8%; titanium between about 0.10 and 0.20%, more preferably between about 0.12 and 0.18%; vanadium between about 0.10 and 0.35%, more preferably 0.10 and 0.25%, zirconium between about 0.10 and 0.30%, more preferably between about 0.10 and 0.25%, and zinc between about 0.10 and 0.40%, more preferably between about 0.10 and 0.30%.
The high iron embodiment has preferred alloying element ranges of: manganese between about 0.2 and 1.5%, more preferably between about 0.3 and 1.2%; zinc between about 0.10 and 0.40%, more preferably between about 0.10 and 0.30%; titanium between about 0.10 and 0.20%, more preferably between about 0.12 and 0.18%; vanadium between about 0.10 and 0.35%, more preferably 0.10 and 0.25%, and zirconium between about 0.10 and 0.30%, more preferably between about 0.10 and 0.25%.
More preferred embodiments include the low iron composition with vanadium alone, manganese alone, or the combination of zirconium and titanium.
For high iron, more preferred embodiments include the use of titanium or titanium and zirconium.
Another aspect of the invention entails a method of brazing one article to another article under elevated temperature and in the presence of a brazing material. The one article is formed from the inventive aluminum alloy composition as described above, whereby the alloying elements of the alloy composition form a fine grain microstructure when subjected to the elevated temperatures due to brazing. Preferably, the one article is bare tubing, and more preferably bare micro-multivoid tubing, and the other article is heat exchanger fin stock or header block(s). For brazing operations, the elevated temperature ranges between about 500 and 650xc2x0 C. In the post-braze state, the tubing, due to its grain growth resistant composition, has a plurality of grains traversing side and end walls to form a fine grained structure that better resists intergranular corrosion effects.
In another aspect of the invention, the inventive aluminum alloy composition is formed into a shape and hot worked by heating the workpiece to an elevated temperature and subjecting the shape to hot working to form an article. In a preferred mode, the hot working is extrusion, and the extruded shape is an elongated hollow article such as tubing. In a more preferred embodiment, the extruded shape is flat micro-multivoid tubing for heat exchanger use. Given the corrosion resistance of the composition, the tubing can be used in its bare condition for brazing. The inventive composition has excellent corrosion resistance, grain growth resistance, and hot workability, making it ideal for use in heat exchanger applications.